Half of Delhi's cars will go off the streets on alternate days from January 1, the Arvind Kejriwal government announced on Friday. Only cars with licence plate numbers ending with odd digits will be allowed to travel on one day and those with even numbers on the next.

The Delhi government’s announcement is its latest move to cut emissions in a city whose emission levels are regularly well above hazardous. In October, the city experimented with a Car-Free Day, which was on a public holiday.

In April, the Supreme Court upheld a National Green Tribunal order that the Delhi government should ensure that vehicles older than 15 years would not be allowed on the roads. It also admonished the government for not taking air pollution seriously and asked it yet again to come up with a viable plan to address these levels.

On Monday, PM 2.5 levels in Anand Vihar in Delhi touched 530 micrograms per cubic metre. This is extremely hazardous to human health.

Premature mortality due to air pollution. Image courtesy: NASA

In an ideal world, Kejriwal's move might seem wise. Half a city's vehicles will go off the streets and pollution levels will plummet. Yet as critics pointed out almost immediately, all this means is that Delhi's car users will come up with workarounds to continue to drive on all days.

How does one encourage people to move out of their cars into public transport or to zero-emission modes of travel? The question is a pressing one that no one city in the world seems to have the answer to. Global examples for almost 30 years have proved that this kind of restriction is meaningless unless vehicles themselves are forced to switch to cleaner emissions.

Here is what some cities have done and how that has worked.

Santiago, Chile
In June, Chile declared its first environmental emergency in 16 years after air pollution levels in Santiago breached its highest air pollution warning level. Chile responded immediately to this disaster. It ordered over 1,000 factories in and around Santiago to stop functioning for 24 hours. Forty per cent of the city’s 1.7 million cars also kept off the roads, based on the last digits of their licence plates and people were advised to avoid exercise outdoors.

Chile is located between two mountain ranges, meaning that, again like Delhi, it is in a basin that traps polluted air. The country expanded its restrictions on vehicles at the end of July. Cars without green licence plates indicating the presence of catalytic converters that neutralise harmful exhaust particles, and those ending in certain digits are now not permitted to ply on roads between 7.30 am and 9 pm.

Paris, France
Vehicle bans are only short-term emergency measures even in developed nations. Responding to alarming air quality readings in March 2014, Paris declared that even-numbered cars would stay off the streets on one day. Before odd-numbered cars could go off streets on the next day, Paris authorities called off the ban, saying that air had improved.

Even that single day of car restrictions was not entirely successful – by noon the police had fined 4,000 drivers for violating the rules and overall use of public transport did not increase.

Mexico City, Mexico
The long-term impact of vehicular bans can actually lead to more cars on the road. One of the first cities to implement traffic restrictions was Mexico City with its Hoy No Circula policy in 1989, which made cars get off the road one day a week based on their licence plate numbers. City authorities hoped that this would encourage people to use public transport on that day while also bringing down the overall level of pollution.

The policy seemed promising at first as pollution levels decreased. Within 11 months, however, emissions went up again, even as people’s spending on travel increased. Instead of fewer cars on the roads, the overall number increased as citizens bought second cars with appropriate number plates to use on their off days. These vehicles were cheaply bought and so often had higher emissions than cars already on the streets. People also began to use taxis more frequently, instead of using low-emission alternatives.

Beijing, China
Beijing is so polluted, an artist has been going around on the roads with a vacuum cleaner and making bricks out of the dust he collects. Yet Beijing also managed to clean up its air pollution in time for the Olympics with drastic measures such as shutting down factories and removing half the city's vehicles from its roads.

It was not able to sustain these measures in the long term. Pollution has once again soared in the city, particularly in winter as smog set in. China is making efforts to move away from coal-based energy to greener sources. It has also shifted highly polluting factories away from the city – where presumably others are welcome to suffer their harmful effects.

London, United Kingdom
In an experiment to give people incentives to shift to public transport or clean up their cars, London implemented a Low Emission Zone policy in 2008, where only those cars that met certain emission standards would be allowed to run in parts of the city where pollution tends to be high. This was an ambitious policy – the zone in London was the largest such in the world.

Heavily polluted vehicles were also fined between 100 and 200 pounds daily. However, the policy was not a success. A study in 2015 found that children in 23 schools near high pollution areas still suffered the same number of diseases as before the low emission policy came into effect. The culprit, experts feel, is probably diesel, which cars are still not able to make less polluting.